The disclosure relates to a laser apparatus.
With miniaturization and high integration of a semiconductor integrated circuit, improvement of resolution is demanded in a semiconductor exposure apparatus. Hereinafter, the semiconductor exposure apparatus is simply referred to as “exposure apparatus”. Shortening of a wavelength of light outputted from an exposure light source is accordingly proceeding. A gas laser apparatus is used for the exposure light source, in place of an existing mercury lamp. Currently, as a gas laser apparatus for exposure, a KrF excimer laser apparatus that outputs ultraviolet rays of a wavelength of 248 nm and an ArF excimer laser apparatus that outputs ultraviolet rays of a wavelength of 193 nm are used.
As current exposure technology, liquid immersion exposure is practically used in which a clearance between a projection lens on exposure apparatus side and a wafer is filled with a liquid to change a refractive index of the clearance, shortening an apparent wavelength of light from the exposure light source. When the liquid immersion exposure is performed with use of the ArF excimer laser apparatus as the exposure light source, ultraviolet rays with a wavelength of 134 nm in water are applied to the wafer. This technology is referred to as ArF liquid immersion exposure. The ArF liquid immersion exposure is also referred to as ArF liquid immersion lithography.
A spectral line width in free oscillation of the KrF and ArF excimer laser apparatus is wide and about 350 pm to about 400 pm. Thus, when the projection lens of any of these laser apparatuses is used, color aberration of a laser beam occurs. This may result in degradation in resolution. It is therefore necessary to narrow the spectral line width (spectrum width) of the laser beam outputted from the gas laser apparatus to an extent that the color aberration is negligible. A line narrowing module including a line narrowing device (such as an etalon and a grating) is thus provided inside a laser resonator of the gas laser apparatus, which achieves narrowing of the spectrum width. The laser apparatus in which the spectrum width is narrowed in this way is referred to as a line narrowing laser apparatus. For example, reference is made Japanese Unexamined Patent Application Publication No. H6-29592, Japanese Unexamined Patent Application Publication No. S63-229789, and U.S. Pat. No. 7,856,044.